This invention relates to a rotational speed control system for an internal combustion engine, and more particularly to a rotational speed control system for controlling a rotational speed of an internal combustion engine in a manner to coincide it with a target rotational speed.
Conventionally, a rotational speed control system which is adapted to coincide a rotational speed of an internal combustion engine with a target rotational speed has been proposed in the art. One type of such a rotational speed control system is disclosed in U.S. Pat. No. 3,724,433, which is constructed so as to differentiate a rotational speed detection signal to obtain a first differential signal and then detect a phase between the first differential signal and a second differential signal obtained by differentiating a target rotational speed signal generated from an oscillator, to thereby coincide the rotational speed with the target rotational speed.
Another type of the conventional rotational speed control system is disclosed in U.S. Pat. No. 4,669,436, which is adapted to prepare a speed deviation signal using a rotational speed detection signal, an accelerator position signal and a droop factor signal and then subject the speed deviation signal to integration to obtain a signal, which is then used for controlling a rac actuator.
Further, Japanese Patent Publication No. 15623/1980 (55-15623) discloses a further type of such a conventional rotational speed control system constructed so as to obtain a pulse signal of which a pulse width is modulated depending on a difference between an actual rotational speed of an internal combustion engine and its target rotational speed. The pulse signal thus obtained is then used for on-off controlling of a drive current fed to an actuator adapted to adjust a rate of fuel fed to the engine. In the rotational speed control system disclosed in the Japanese publication, an integral signal obtained by integrating a difference between a rotational speed detection signal and a temperature detection signal is compared with a sawtooth signal voltage in a comparator, resulting in the pulse signal for driving the actuator being obtained. Unfortunately, the control system disclosed fails to permit an output of the comparator to be varied during a length of time for which the integral voltage is kept between a maximum value of the sawtooth signal voltage and a power supply voltage and between a minimum value of the sawtooth signal voltage and 0 V, so that a dead time or dead section occurs in controlling of the rotational speed. This causes response to the controlling to be delayed, resulting in overshoot being increased.